This invention relates to a liquid crystal device.
Liquid crystal devices typically comprise a thin layer of a liquid crystal material sandwiched between substrates acting as cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer, causing a re-alignment of the liquid crystal molecules.
Many different modes of liquid crystal devices are known in the art. Examples of known device types are the twisted nematic device, the cholesteric phase change device, the dynamic scattering device, the supertwisted nematic device and the surface stabilised ferroelectric device. It is well known in all of these device modes to provide a surface on the interior walls of the device which will control the alignment of the liquid crystal fluid in close proximity to the surface. The surface or surfaces may provide only one stable alignment of the liquid crystal within the cell, in which case the device is termed xe2x80x9cmonostablexe2x80x9d. If two or more zero-field alignments are afforded, the device is termed xe2x80x9cmultistablexe2x80x9d. In general where only two stable alignments, whose optical properties are significantly different, are obtained, devices are termed xe2x80x9cbistablexe2x80x9d.
The class of bistable nematic devices can be sub-divided into xe2x80x9cazimuthalxe2x80x9d, xe2x80x9czenithalxe2x80x9d and xe2x80x9ctwistedxe2x80x9d types. In xe2x80x9cazimuthalxe2x80x9d types, the switching between the stable states requires a general rotation of the liquid crystal bulk director in the plane of the substrates, whereas in xe2x80x9czenithalxe2x80x9d types this movement is in a plane normal to the substrates surface. For xe2x80x9ctwistedxe2x80x9d types, the movement between states is characterised by a significant change in the nematic director twist angle through the device about an axis normal to the substrates.
The liquid crystal may be realigned by use of electric fields applied between the electrodes on the cell walls. For liquid crystals with positive dielectric anisotropy, the result is alignment parallel to the field, whereas for liquid crystals with negative dielectric anisotropy, the result is alignment normal to the field.
For bistable and multistable devices, a key issue is switching between two different stable statesxe2x80x94this should not occur uncontrollably, but should be reliable, repeatable, and should require little power. Switching is accomplished by realigning liquid crystals through the electric fields between electrodes. In standard cells, this realignment is a bulk effect, and it requires higher electric fields than is desirable to work efficiently.
Other approaches have been tried. Ohta et al, in xe2x80x9cDevelopment of Super-TFT LCDs with In Plane Switching Display Modexe2x80x9d, IDRC 95, p707, proposes use of two different electrodes interdigitated on a single substrate. International Patent Publication No. WO 81/01618 teaches use of different electrodes on a single substrate to achieve more than one field geometry. Such approaches have had problems in effective application. The electrode density is important for in-plane switching to be effective, but it is limited by the tendency for shorting between opposed conductors to occur, because such shorting results in the failure of the pixel concerned.
A further modification beneficial in aligning liquid crystals at a cell surface is to introduce topographic features of a microscopic scale. This approach has been followed for monostable alignment (discussed in Cheng, J., Boyd, G. D., Applied Physics Letters, Vol. 35, p1326, 1970) and for bistable alignment (see European Patent Publication No. 0744041). Use of a grating structure is discussed in xe2x80x9cThe Alignment of Liquid Crystals by Grooved Surfacesxe2x80x9d, D. W. Berreman, Mol. Liq. Cryst. 23, pp 215-231, 1973. Use of such topographic features allows control of the ordering of the liquid crystal at the surface. More recently (as summarised in Konovalov et al, Asia Display 98, p379), the dielectric effect of larger scale surface features on the field geometry has been used to allow consistent alignment of Vertically Aligned Nematic (VAN) modes.
A key problem still remains in that in conventional arrangements, the effect of an applied electric field is realised mainly in the bulk, and a large bulk effect is required to change the alignment of the director near or at the surface. Consequently high voltages are required to generate such high fields across the cell, and control of switching could be usefully improved.
Accordingly, the invention provides a liquid crystal display cell, comprising: a liquid crystal layer having a thickness dimension and a bulk region bounded by first and second limits of extent of the liquid crystal layer in the thickness dimension; first and second electrode structures disposed for orienting liquid crystals in the liquid crystal layer; wherein said first and second electrode structures are displaced with respect to each other in said thickness dimension, and wherein at least one of the first and second electrode structures lies within the bulk region.
This has a number of advantages. As the vertical gap between electrodes is small, low drive voltages are needed to give a significant field between them (moreover, in preferred constructions a barrier layer can readily be used to protect the electrodes from each other). Moreover, this field is located in the region in which it can be particularly effectivexe2x80x94adjacent to the surface. Electrodes can be patterned at high resolution without risk of shorting (because of the vertical displacement). Considerable control can be made of the topology of electric fields resulting, which can have important significance as the relationship of electric field direction to liquid crystal director is of central importance to effective switching. This relationship provides the torques experienced by the liquid crystal, and as these torques are provided near the surface (rather than in the bulk of the liquid crystal), they are faster acting and more effective. Yet a further advantage is that designs can be made which result in little or no electric field acting on the bulk, which can be beneficial as electric fields can affect liquid crystal optical properties.
Preferably, the first electrode structure lies adjacent to the bulk region and the second electrode structure lies within the bulk region: a particularly practical arrangement is for the first electrode structure to be formed on a first substrate and lie between the first substrate and the bulk region, with the second electrode formed on a raised structure formed on the first substrate and extending into the bulk region. This raised structure may have the same role as existing microscopic topographic structures do, of aligning the liquid crystal. The raised structure can thus be an array of ridges, but a new and useful alternative is to provide the raised structure as a layer perforated by an array of holes extending to the first electrode structure.
There may also be a third electrode structure on the opposite cell surface, and even a fourth electrode structurexe2x80x94the third and fourth electrode structures may have essentially the same types of features of first and second electrode structures. A useful approach, if there are first, second, third and fourth electrode structures, is for orthogonal liquid crystal alignments (by raised structures for liquid crystal alignment or otherwise) to be favoured at the two surfaces.
The present invention is particularly useful for nematic liquid crystals, both in cells that have a monostable or bistable (either zenithal or azimuthal) alignment without electric field.